Cryogenic helium flow cryostats have been used for many years to regulate temperature in systems designed to test the physical properties of laboratory specimens. The need for testing physical specimens has increased substantially over the last several years. These systems are designed to characterize the physical properties of various materials under variable measurement conditions. Furthermore, these systems are capable of being programmed for an arbitrary sequence of temperature, magnetic field sweeps, and steps at which to characterize various physical properties of the sample specimen.
It is often necessary to control the temperature of these specimens precisely over a wide range of temperature from liquid helium temperatures to well above room temperature. The instruments used for characterization often contain a number of massive components, including superconducting magnets and other cryogenic components, which, because of their mass are prohibitively time-consuming to cool-down and warm-up, or require being maintained cold in order to function. In this case, it is necessary to cycle the temperature of only the specimen or a relatively small portion of the cryostat surrounding the specimen, while the other cryogenic components, such as the superconducting magnet, are maintained at an operational cold state.
This combination of requirements has led to the development of gas-flow cryostats where the specimen is cooled by flowing refrigerated helium gas over the specimen, or the space surrounding a sealed chamber containing the specimen. The refrigerated helium gas provides the cooling power, while electrical heaters attached to the chamber provide the ability to warm the specimen. It may be necessary to vary both the flow rate and heater power to sweep and control the temperature of the specimen. Rapid thermal cycling of the specimen is possible because it is only necessary to warm the helium gas and chamber in the vicinity of the specimen. It is not necessary to warm or cool the other components in the cryostat, including the source of refrigerated helium.
The coldest possible temperatures in a gas-flow cryostat are achieved by using the vapors from boiling liquid helium as the source of refrigerated helium gas. A vacuum pump may be used to simultaneously pump on a small reservoir of liquid helium and to draw the evolving vapors over or around the specimen region in the cryostat. Because the vapors are at the same temperature as the boiling helium (typically at 1 to 2 K for the helium-4 isotope); the specimen can be cooled to near the temperature of the boiling helium. As mentioned above, because the specimen is cooled by the evolved vapors and not directly by the liquid, it is possible to quickly warm the specimen with only the use of the variable heaters without the need to warm the liquid bath.
Continuous operation is achieved by continuously filling the evaporation reservoir with liquid at low pressure using a capillary or other flow restrictor. This liquid is provided either by a larger bath of commercially liquefied helium at atmospheric pressure, or by liquefying a room-temperature helium gas stream using a cryogenic refrigerator such a Pulse Tube (PT) or Gifford-McMahon (GM) cryocooler. In a recirculating design, the room-temperature helium gas comes from the exhausting helium gas flowing from the cryostat through the pumping system.
Presently available designs of continuously filled, pumped-helium gas-flow cryostats use a variety of techniques for restricting the flow of liquid into the evaporation reservoir. One type uses a fixed capillary or orifice as the flow restrictor, See Delong et. al., “Continuously Operating He Evaporation Refrigerator”, The Review of Scientific Instruments, Vol. 42, No. 1, January 1971. The geometry of this capillary is optimized to provide a specific flow rate. Flow too high can overfill the reservoir, or increase the vapor pressure and hence the boiling temperature. Flows that are too low may provide insufficient cooling power to the specimen, or cause the reservoir to run dry, and thus a sudden loss of cooling. Another type of flow restrictor in the art uses a cryogenic mechanical valve that can be adjusted in situ to change the liquid flow rate into the evaporation reservoir. Though less common, it is also possible to use a fixed-geometry restrictor, such as a capillary, in combination with attached heaters to change the effective flow impedance of the capillary by changing the temperature and hence viscosity of the helium flowing in it.
The rate at which helium gas is evaporated from the reservoir is determined by the vacuum provided by the pumping system, the geometry of the pumping lines, and the heat-load on the reservoir from inflowing liquid, parasitic heat sources, or evaporation heaters attached to the reservoir.
In these systems, the inflow rate and outflow rate are selected such that the evaporation reservoir does not dry out, and also so that the flow is not so high as to overwhelm the pumping system and thereby increase the minimum temperature. In the case where the filling rate exceeds the evaporation rate, the liquid level will rise until the inlet is starved of liquid, as might be the case with a closed recirculating system with a finite charge of helium or other cryogen gas, or until parasitic heat from the chamber heaters or the warmer regions of the cryostat increase the evaporation rate to match the inflow rate. As this happens, the flow rate and hence the cooling power available to the specimen chamber varies considerably in time as filling occurs.
In the case where the equilibrium liquid level depends on heat from the specimen, the level can vary with specimen temperature if the accumulated liquid level is sufficiently close to the specimen chamber/exchanger region. For example, if the specimen chamber is allowed to cool to near the boiling point of the helium, there may be very little heat from the chamber and the liquid level could rise, thus increasing the thermal coupling between the liquid and the specimen chamber. Once the liquid level has risen, application of heat to the specimen, as is done when routinely increasing the specimen temperature, results in transfer of heat into the liquid and thus increased boil off and cooling power. This increased cooling power will require a compensating increase in the heater power needed to affect a given temperature rise. The increased boil off will reduce the level of the liquid over time, and thus decrease the cooling power, resulting in poor temperature stability.
Thus, while gas-flow cryostats are very capable for achieving rapid temperature changes over a wide range of temperatures down to pumped liquid helium temperatures, the lack of liquid level and flow-rate control limits the temperature stability that is possible with such systems. This is a significant limitation for many applications, such as specific heat or thermal conductivity measurements, where small fluctuations in the temperature of the specimen and experiment region can lead to significant errors in the physical property measurement.